Electronic oven



Dec. 6, 1966 P. H. SMITH 3,289,570

ELECTRONIC OVEN Filed Oct. 19, 1964 5 Sheets-Sheet l Wm M 677% may;

Dec. 6, 1966 Filed Oct. 19, 1964 w v 52 llnmf" 8 .ml m. -.%1 mm..-

um. will! I- nllllm. I HIIIHIZI- X LLU 5 Sheets-Sheet 2 P. H. SMITH ELECTRONIC OVEN Dec. 6, 1966 Filed Oct. 19, 1964 .3 Sheets-Sheet 3 m! II HIHIII AHI llih MICRO WAVE SOURCE TIMER SWITCH SWITCH SOLENOID 44 3,289,570 Patented Dec. 6, 1966 3,289,570 ELECTRONIC OVEN Peter H; Smith, Maidenhead, England, assignor to .l.

This invention relates to electronic ovens and more particularly to a semiautomatic electronic oven specifically adapted for heating or cooking frankfurter sandwiches or hot dogs.

Several disadvantages have been inherent in prior art ovens for cooking sandwiches or the like, and one of the major disadvantages of such ovens is connected with the relatively large number of functions which are necessary for the operator to perform in using the oven to heat or cook food. No less than five operator functions are ordinarily required, which involve inserting the food to be heated, closing the door, starting the oven, and, after the heating operation is completed, opening the door and removing the food. On some ovens it is also necessary to set a timer to preprogram the length of the heating operation. Hence, it is desirable to eliminate some of the functions which the operator must perform, to render the use of the oven more efiicient.

Another disadvantage of prior art ovens which is peculiar to the electronic variety, is the danger of initiating operation of the oven when there is no food within the heating chamber of the oven. When, as usual, a magnetron is employed as a source of microwave electromagnetic energy in an electronic oven, a severe mismatch between the load impedance as seen by the magnetron and the source impedance, occurs during operation of the oven when there is no food in the heating chamber, and produces a high standing wave ratio reflecting energy back to the magnetron. The magnetron may be destroyed by the reflected energy if the standing wave ratio is too high. Hence, it is important to insure that the oven cannot be operated unless food is present within the heating chamber.

One way of preventing operation of the oven without food, which has been attempted in the prior art, is to provide a movable platform on the floor of the heating chamber of the oven which is adapted to actuate a switch in response to the weight of food, and its container, resting on the platform. It is obvious, however, that with such an arrangement, the weight required to operate the switch is merely greater than a predetermined value, and even an empty container having a greater weight than the predetermined value would permit operation of the oven. It is desirable, therefore, to provide more effective means of accomplishing this result.

Another disadvantage of prior art electronic ovens is that it has heretofore been necessary to compromise between factors involving ease of using the oven, and other factors insuring a constant load impedance and uniform heating of the food. The load impedance, and also the uniformity of the electromagnetic field within the heating chamber, are somewhat dependent upon the location of food within the heating chamber. Ideally, the heating chamber is of such size and configuration as to just receive the package containing the food to be heated, with substantially no remaining unoccupied space. Thus the food can take only a single position within the heating chamber, without any positional variation. The better the food package fits into the heating chamber, however, the more diflicult it is to remove the package after it is heated. Hence, it is desirable to provide an oven which is adapted to receive a standard size food package with provision for facilitating its removal from the oven.

Another disadvantage of the electronic ovens of the prior art involves the necessity of sealing the oven chamber so that no radiation can escape from the chamber when it is energized. Many attempts have been made in the past to provide such sealing means, and the most effective heretofore have been of the type which are hinged on one edge, and adapted to swing outwardly to permit access to the heating chamber. This type of door requires more space for the oven, so that the door may swing open to provide access to the chamber, and also ordinarily requires a latch by which the door is positively held shut. One disadvantage of such a door is that the surfaces on the door and the oven close in faceto-face relationship, and any dirt or foreign matter which may be present between the facing surfaces may tend to maintain the surfaces slightly spaced apart at that point, and allow the escape of radiation. Moreover in a hinged door which employs resilient means to provide a tight mechanical closure, substantially more effort is required to close it. In addition, opening and closing the door causes repeated flexing and unfiexing of the resilient means, so that relatively large springs must be provided to minimize the effects of metal fatigue over a long duration. It is therefore desirable to provide a door which is not subject-to these disadvantages.

It is a principal object of the present invention to eliminate several of the required operator functions in the use of electronic ovens, and to make the same more automatic. In the operation of the apparatus of the present invention, it is only necessary for the operator to insert the food into the heating chamber and close the door, the other functions of the apparatus thereafter being carried out completely automatically. Upon closing the door, the oven automaticaly starts to heat the food for a predetermined period of time, after which the heating stops, the door opens, and the food is partially ejected from the oven into a position convenient for the operator.

Another object of the present invention is to provide means to permit operation of the oven when food is placed within the heating chamber thereof, irrespective of the weight of the food or its container.

A further object of the invention is to provide movable means displacing a portion of the volume of the oven chamber when no food is present therein, and the placement of food therein functioning to enlarge the volume of the heating chamber by displacing such movable means.

A further object of the present invention is to provide an oven having a heating chamber which is substantially the same size and shape as the food package to be heated therein.

Another object of the present invention is to provide means for automatically partially ejecting heated food from the heating chamber after the termination of the heating period.

These and other objects and advantages of the present invention will become manifest by a consideraton of the specification and the accompanying drawings.

One embodiment of the present invention comprises an oven which is peculiarily adapted for heating hot dogs which are contained within fibrous tubes or cartridges. The heating chamber of the oven is of an appropriate size and shape as to receive only one of such cartridges at a time. The heating chamber is provided with movable means connected to an associated switch. The switch inhibits operation of the oven except when the movable means is displaced by insertion of food in the heating chamber. Means is provided to partially eject the food from the heating chamber after the end of the heating period.

A source of microwave radiation is provided to heat or cook food disposed within the heating chamber. A hingeless door is provided in slidable relationship to an end wall of the oven chamber, with means for automatically opening the door after the end of the heating period. A switch is responsive to the position of the door to inhibit operation of the oven except when the door is in closed position.

Reference will now be made to the accompanying drawings in which:

FIG. 1 is a cross-sectional view of a portion of an oven embodying the present invention;

FIG. 2 is a front view of a portion of the apparatus illustrated in FIG. 1;

FIG. 3 is a cross-sectional view of the apparatus illustrated in FIG. 1 taken along the line 33;

FIG. 4 is a side elevational view of the apparatus illustrated in FIG. 3;

FIG. 5 is a plan view of the apparatus illustrated in FIG. 1;

FIG. 6 is a partially exploded perspective view of the door assembly of the present invention;

FIG. 7 is a side elevation of a pawl mounted on the door panel;

FIG. 8 is a plan view of the gear housing associated with the apparatus of FIG. 1;

FIG. 9 is a perspective view of a food package designed for use in an oven incorporating the present invention; and

FIG. 10 is a functional block diagram of the electrical control circuitry associated with the apparatus in FIG. 1.

Referring now to the drawings, there is shown in FIGS. 1 and 5 a metal oven compartment 10 having rectangular upper and lower walls 14 and 72, respectively, and two pairs of facing side walls 73 closing the compartment 11). Within the compartment 10 is a centrally disposed circular cylindrical tube 12, which tube forms the sides of a heating chamber within the oven compartment 19. The entire oven compartment is preferably surrounded by a casing 7, partly shown in FIG. 1. A waveguide 13 is connected to the oven compartment 11) on one side thereof, and is adapted to be connected to a magnetron (not shown) at a coupling 15. A tuning stub 21 is provided for tuning the waveguide in the well known manner. The relationship of the oven compartment 10, the heat ing chamber 12, the waveguide 13, and the magnetron are preferably as described in co-pending Peter H. Smith application Serial No. 293,685, filed July 9, 1963. A baflle such as is described and claimed in that application is preferably employed in the oven compartment of the present invention.

The tube 12, which is described in the aforementioned co-pending application as being formed of low loss dielectric material, is open at both ends, and at its upper end, as illustrated in FIG. 1, there is an aligned opening 19 of an upper wall 14 of the oven compartment 10. An annular bearing 21 is mounted in the opening 19 and supports the upper end of the tube for rotation relative to the upper wall 14. An opening 9 in the outer casing 7 is also aligned with the opening 19. Mounted on the upper wall 14 are a pair of parallel guides 16 and 17 (FIG. 5) which guide a door 18 is in slidable relationship therewith. The door 18 has a pair of elongate slides 20 slidably engaged in grooves 135 in the guides 16 and 17. A resilient pad 24 formed of resilient foam plastic such as polyurethane or the like is secured to the interior surface of the door 18 by adhesive or the like and a thin resilient metal sheet 26 is secured to the interior surface of the pad 24. The pad 24 resiliently urges the sheet 26 against the upper wall 14 of the oven chamber 10. As the pressure on the sheet 26 is evenly distributed by the pad 24 over the surface area thereof, the sheet 26 is in substantially continuous contact with the upper wall 14 except for the opening 19.

A handle 28 is provided on the exterior surface of the door 13, and by this means the door 18 may be slidably moved in a direction parallel to the upper wall 14 by sliding the slides 20 within the guides 16 and 17.

The door 18 is resiliently urged into its open position by a spring 30 secured between the door 18 and a lug 32 mounted on the guide 16 (FIG. 6). The spring 30 is disposed in an elongated notch 31 provided in the guide 16 just above the groove 135. There is secured to the front portion of the door 18, a U-shaped member 34 forming a latch bar just below an aperture 35 in the door 18. The latch bar 34 cooperates with a latch arm 36 to selectively hold the door 13 in closed position, with the sheet 26 closing the opening 19. The latch arm 36 is pivotally mounted on a shaft 38 which, in turn, is mounted on the end wall 14 by a bracket 40 (FIGS. 2 and 5). The end of the latch arm 36 which is remote from the latch bar 34 is pivoted to the operating arm 42 of a solenoid 44. A wire spring 46 (FIG. 1) disposed on the shaft 38, has one end in engagement with the bracket 40 and the other end in engagement with the latch arm 36, and functions to urge the latch arm 36 clockwise as illustrated in FIG. 1. When the solenoid 44 is energized, the latch arm 36 pivots counterclockwise about the shaft 38, and releases the latch bar 34 to permit the spring 36 to return the door 18 into its open position, uncovering the opening 19 to permit access to the heating chamber.

A pair of brackets 48 (FIG. 2) are secured to the solenoid 44 and support a microswitch 5t? thereon. The microswitch 50 has an actuating arm 52 which is engageable by a pin 53 mounted on the forward end of one of the slides 20, when the door 13 is in closed position as illustrated in FIG. 1. Thus, the microswitch 50 is actuated when the door 18 is in closed position, but is unactuated at all other times. The condition of the switch 5t thereby indicates the condition of the door 13, and, as will be more fully described hereinafter, prevents energization of the oven when the door 18 is opened.

A piston 56 (FIG. 1) is mounted within the tube 12 and is provided with a circular platform 58 having its periphery in fitting engagement within the tube 12. The piston 56 is connected to a shaft 60 which extends downwardly away therefrom. The shaft 60 is provided with a circumferential notch 62, which cooperates with latch means to be described hereinafter. The bottom portion of the shaft 60 is hollow, and contains a coil spring 64 operating between the upper portion of the shaft 60 and the inside surface of an end wall 66 of a tubular housing 63. The tubular housing 68 is secured to a gear housing 70 which, in turn, is mounted on the lower wall 72 of the oven compartment 10. The gear housing '70 is constructed of metallic material and houses a pair of gears which will be more fully described hereinafter. A rod 76 is secured to the housing 68 in coaxial relation therewith, by a threaded shaft 74 threadably engaged with the rod 76 and the end wall 66. The rod 76 functions to align the spring 64 to prevent it from becoming fouled within the shaft 60.

A flange 73 is provided at the lower end of the shaft 69. When the piston 56 is in its lower position, the flange 78 engages a pin 30 disposed in an aperture in the end wall 66 of the housing 63. The pin 80 serves as the operating pin of a microswitch 82 mounted on the end of the tubular housing 68 by an L-shaped bracket 84.

In FIG. 9 there is shown a tubular food-containing cartridge 69 intended for use with the apparatus of the present invention. The diameter of the cartridge 69 is such as to easily slide into the tube 12, without leaving any substantial space at the sides between the tube 12 and the cartridge 69. The length of the cartridge 69 is such that when it is fully disposed inside the heating chamber, the flange 78 engages the pin 80 and operates the microswitch 82. Thus, the switch 82 is actuated only if the piston 56 has been displaced by insertion of a food container into the tube 12-, thereby depressing the platform 58. The force of the spring 64 is sufliciently great to prevent the depression of the platform 58 by the weight of the food and its container alone. The food container must be actively forced by hand pressure into the heating chamber before the switch 82 can be actuated.

The tubular housing 68 is provided with a slot 71 (FIG. 1) near the gear housing 70, which permits a latch arm 86 to enter the interior of the tubular housing 68. The latch arm 86 is pivotally mounted at 88 to the gear housing '70 (FIG. 3), and is provided with a projection or tab 90 adapted to engage the groove 62 of the shaft 60. An L-shaped member 91 is provided with a slot 93 (FIG. 4) which holds the latch arm 86 in a plane parallel to the gear housing 70. The latch arm 86 is urged in a clockwise direction, as illustrated in FIG. 3, by a spring 92 connected between a hole 94 in the end of the latch arm 86, and a stud 96 on the bracket 70. The spring 92 urges the projection 90 of the arm 96 into latching engagement with the groove 62 so that when the piston 56 has been displaced sufficiently to bring the groove 62 into juxtaposition with the tab 90, the piston 56 thereafter cannot be returned to its initial position until the latch arm 86 has been unlatched.

The unlatching mechanism for the latch arm 86 includes a link 98 having one end pivotally connected by a stud 100 to the latch arm 86, and pivotally connected by a stud 102 at its other end to a lever 104. The stud 102 is mounted at an intermediate portion of the lever 104, and the lower end of the lever 104 is pivotally secured to a bracket 106 by a stud 108. The bracket 106 is secured to the gear housing '70 by machine screws 110.

The other end of the lever 104 extends upwardly to a position spaced outwardly from the plane of the end wall 14 of the oven chamber 10. The lever 104 is adapted to e engaged by a pawl 112 (FIGS. 1, 5 and 7), pivotally mounted by a stud 116 on a bracket 114. The bracket 114 is secured to the rear end of the door 18. The construction of the pawl 112 and its supporting bracket 114 is such that the limit of its rotation in a clockwise direction is illustrated in FIG. 1. However, the pawl 112 may be rotated in a counterclockwise direction for more than 90. As illustrated in FIGS. 5 and 7, the bracket 114 is generally U-shaped and has a pair of flanges 115 by which it is secured to the door 18. A slot 117 is provided in the lower portion of the outer wall to permit counterclockwise rotation of the pawl 112. The upper portion of the pawl 112 engages the outer wall of the bracket 114 above the slot 117 to prevent clockwise rotation.

As the end 118 of the lever 104 is disposed in the path of travel of the pawl 112, as the door 18 moves along its guides 16 and 17, the end 118 of the lever 104 is moved backwardly by the pawl 112 until the latter passes the lever 104 by riding over it. When this occurs, the lever 104 returns to its initial position under the force of the spring 92. In the meantime, however, the tab 5 0 is momentarily withdrawn from the engagement with the groove 62 of the shaft 60, to permit the piston 56 and its platform 58 to move to their upper positions within the tube 12 under the force of the spring 64. The upper position of the platform 58 is reached when the flange '78 strikes a stop member 105 comprising a tubular portion surrounding the shaft 60, and a flange at the upper end of the tubular portion held in position between the gear housing 70 and the tubular housing 68. The length of the tubular portion of the stop member determines the upper position of the platform 58. The upward travel of the platform 58 partially ejects the food within the heating chamber. The lever 104 is not engaged by the pawl 112, however, for delatching of the piston 56 until the door 18 has reached its open position, so that there is no obstruction to the food being ejected from the heating chamber.

As has been described, the pawl 112 rides over the end of the lever 104 during its rearward travel, and the lever 104 springs back to its initial position. Upon closing of the door 18, however, the pawl 112 engages the end 118 of the lever 104 with its inclined front edge 120, and rotates about the stud 116, thereby riding over 6 the end 118 of the lever 104 without disturbing the position of the lever 104.

Within the gear housing 70, there is disposed a drive gear 122 and a main gear 125 (FIG[ 8). The drive gear 122 is connected by a universal joint 124 (FIG. 1) to the drive shaft 126 of a motor 128 which is mounted on the gear housing 70 by a bracket 130.

The gear housing is provided with a pair of overlapping circular recesses 131 and 133 to accommodate the gears 122 and 125, and the walls of the recesses serve as bearings for the gears. The drive gear 122 is in mesh with the main gear 125, and the main gear 125 is secured to the tube 12 so that the tube and the food within it rotate during the cooking process. The platform 58 fits sufficientiy tightly within the tube 12 so that friction therebetween causes the platform 58 to rotate with the tube 12.

In the operation of the apparatus of the present invention, the cartridge 69, containing food to be heated, is inserted into the heating chamber and pushed downwardly to cause the piston 56 to be latched in its lower position by means of the tab 90 engaging the groove 62. The door 18 is then pulled forwardly to enable the latch arm 36 to engage with the latch bar 34, to hold the door 18 in its closed position over the opening 19 which opens into the heating chamber.

The latching of the piston 56 in its downward position causes the flange 78 to engage the pin and actuate the microswitch 82, and the latching of the door 18 in its closed position causes the pin 53 to engage the arm 52 of the microswitch 50 to actuate the latter. The control circuitry for the source of microwave radiation (FIG. 10) includes the switches 50 and 82 in series, so that the oven may not be operated until both switches are actuated.

When both the switches 50 and 82 are closed, the magnetron or other source of wave energy is energized and remains energized for a predetermined time, dependent upon the operation of a timer which is part of the control system for the microwave source. The timer may be of any available type such as, for example, that described in Timm Patent No. 2,831,419, issued April 22, 1958. The details of the timer itself form no part of the present invention. At the end of a predetermined time, an electrical signal is produced by the timer and is applied to the solenoid 44. The solenoid 44 then draws down its actuating arm 42, to cause the latch arm 36 to unlatch from the latch bar 34. This permits the spring 30 to slide the door 18 to its open position, and after the door clears the opening of the heating chamber, the pawl 112 engages the end 118 of the lever 104 to withdraw the tab from engagement with the slot 62 of the shaft 60 of the piston 56. This enables the spring 64 to return the piston 56 to its initial position within the heating chamber, thereby partially ejecting the food cartridge from the heating chamber through the end thereof, which has already been opened by the movement of the door 18.

The present invention thus provides a simple and automatic apparatus for cooking any food disposed within the hollow cylindrical tube or cartridge, such that the same can be used to displace the piston 56 during loading of the food to be heated into the heating chamber.

From the foregoing, the present invention has been described with sufficient particularity to enable others skilled in the art to make and use the same and, by applying current knowledge, to adapt the same for use under varying conditions without departing from the essential features of novelty involved, which are intended to be defined and secured by the appending claims.

What is claimed is:

1. A microwave oven comprising a closed oven chamber defined by metallic walls, a cylindrical heating chamber disposed within said oven chamber and having an open end aligned with an opening in a wall of said oven chamber, metallic door means including a resilient sheet rectilinearly slidably mounted on said wall for movement between a first position covering said open end and a second position uncovering said open end, and means disposed on the opposite side of said sheet from said wall for urging substantially all of said sheet into contact with said wall, means urging said door toward said second position, and latch means selectively holding said door in said first position.

2. Apparatus according to claim 1, including means for selectively releasing said latch means.

3. Apparatus according to claim 1, including switch means, and means for actuating said switch means when said door is in said first position.

4. An oven comprising a metal wall having an aperture therein, a cylindrical heating chamber disposed normal to said wall with an open end aligned with said wall, door means slidable along said wall between a first position covering said open end and a second position uncovering said open end, means urging said door means toward said second position, a piston disposed within said cylinder for slidable movement between first and second positions, means latching said piston in said first position, means urging said piston toward said second position, and means responsive to said door means moving from said first to said second positions for unlatching said latch means.

5. Apparatus according to claim 4 including means for selectively holding said door in said first position.

6. In an oven having heating means, means for energizing said heating means, and timing means for generating an electrical signal a predetermined time after said heating means has been energized, the combination comprising a metal wall having an aperture therein, a cylindrical heating chamber disposed normal to said wall with an open end aligned with said wall, door means slidable along said wall between a first position covering said open end and a second position uncovering said open end, holding means for holding said door in its first position, means urging said door means toward said second position, a piston disposed within said cylinder for sliding movement between first and second positions, means latching said piston in said first position, means urging said piston toward said second position, means responsive to said door means moving from first to said second positions for unlatching said latch means, and means responsive to said electrical signal for disabling said holding means.

7. Apparatus according to claim 6, wherein said disabling means comprises a solenoid.

8. Apparatus according to claim 6 including first switch means for detecting when said door is in said first position and second switch means for detecting when said piston is in said position, said energizing means being responsive to both said first and second switch means.

9. A microwave oven having a heating chamber, means for selectively supplying wave energy to said heating chamber, said heating chamber being defined by a dielectric cylinder, a cylindrical food container adapted to fit closely within said heating chamber but being freely slid..ble therein, means for closing said heating chamber means responsive to said last named means and to the insertion of said food container into said heating chamber for causing said supplying means to supply said wave energy to said heating chamber, and means for automatically partially ejecting said food container from said heating chamber.

10. Apparatus according to claim 9, wherein said last named means comprises a piston slidably disposed within said heating chamber and adapted to be displaced by the insertion of said food container into said heating chamber, resilient means for urging said piston into said cylinder, and means for selectively holding said piston in displaced position.

11. In an electronic oven having an oven compartment with an opening therein to allow insertion and withdrawal of food to be heated, the combination comprising a pair of parallel guides mounted on the exterior surface of a wall of said oven compartment, each of said guides having a longitudinal slot facing the opposite guide, door means disposed parallel to said wall and having a pair of parallel slides secured to opposite edges thereof, said slides being disposed in said slots and prevented from movement thereby except in a longitudinal direction, a resilient conductive sheet mounted on the side of said door facing said wall, and means for urging said sheet into substantially continuously contact with said wall.

12. Apparatus according to claim 11, wherein said last named means comprises a resilient pad of polyurethane secured between said door and said sheet.

13. Apparatus according to claim 11, wherein said door is slidable with respect to said guides between a first position in which said sheet covers said opening and a second position in which said sheet uncovers said openings, and including means for resiliently urging said door toward said second position.

14. Apparatus according to claim 13, including latch means to selectively hold said door in said first position, and means for unlatching said latch means to permit said door to slide to said second position.

References Cited by the Examiner UNITED STATES PATENTS 2,075,474 3/1937 Scutt 99-337 2,274,325 2/1942 Ford 99332 2,329,937 9/1943 Orkfritz 99327 X 2,465,577 3/1949 COX.

2,497,205 2/1950 Brewton 99327 X 2,630,062 3/1953 Litt 99327 2,820,127 1/1958 Argento et al 219-10.55 2,843,410 7/1958 Barnhart 99332 X 3,091,172 5/1963 Wildemann 99427 3,181,452 5/1965 Zaidener 99--332 3,229,613 1/1966 Matzenauer 99-327 l/VALTER A. SCHEEL, Primary Examiner.

BILLY J. WILHITE, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 3,289,570 7 December 6, 1966 Peter H. Smith It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 5, line 53, strike out "the", first occurrence;

column 6, line 15, for "sufficientiy" read sufficiently column line 55, before "position" insert first column 8, line 28, for "continuously read continuous Signed and sealed this 26th day of September 1967.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

6. IN AN OVEN HAVING HEATING MEANS, MEANS FOR ENERGIZING SAID HEATING MEANS, AND TIMING MEANS FOR GENERATING AN ELECTRICAL SIGNAL A PREDETERMINED TIME AFTER SAID HEATING MEANS HAS BEEN ENERGIZED, THE COMBINATION COMPRISING A METAL WALL HAVING AN APERTURE THEREIN, A CYLINDRICAL HEATING CHAMBER DISPOSED NORMAL TO SAID WALL WITH AN OPEN END ALIGNED WITH SAID WALL, DOOR MEANS SLIDABLE ALONG SAID WALL BETWEEN A FIRST POSITION COVERING SAID OPEN END AND A SECOND POSITION UNCOVERING SAID OPEN END, HOLDING MEANS FOR HOLDING SAID DOOR IN ITS FIRST POSITION, MEANS URGING SAID DOOR MEANS TOWARD SAID SECOND POSITION, A PISTON DISPOSED WITHIN SAID CYLINDER FOR SLIDING MOVEMENT BETWEEN FIRST AND SECOND POSITIONS, MEANS LATCHING SAID PISTON IN SAID FIRST POSITION, MEANS URGING SAID PISTON TOWARD SAID SECOND POSITION, MEANS RESPONSIVE TO SAID DOOR MEANS MOVING FROM FIRST TO SAID SECOND POSITIONS FOR UNLATCHING SAID LATCH MEANS, AND MEANS RESPONSIVE TO SAID ELECTRICAL SIGNAL FOR DISABLING SAID HOLDING MEANS. 